Fabrication method for fabricating an object as a plurality of successive laminae

ABSTRACT

A fabrication method in which an object is formed as a plurality of successive laminae, the method comprising the repeated steps of: (i) applying a precursor onto a recipient surface; and (ii) locally heating regions of the deposited precursor by directing a light beam onto those regions of the deposited precursor, so that the locally heated regions transform to a solid material.

This is a continuation of copending International ApplicationPCT/GB98/00414 having an international filing date of Feb. 10, 1998.

This invention relates to methods and apparatus for the fabrication ofobjects.

In the current competitive domestic, industrial and military markets,the commercial lifetimes of products are continually decreasing andtheir design complexity is increasing. This has led to anever-increasing need for rapid prototyping technology in themanufacturing industries. Rapid prototyping has already demonstrated itsability to reduce a product's development time, and to allowmanufacturers to react faster and more flexibly to meet the demands ofthe rapidly changing markets.

At present, there are various types of commercially available rapidprototyping methods, such as (a) selective laser curing of liquidmonomers, usually within a pool of the liquid (Stereo lithography, SLA)[see publication reference 1], (b) selective laser sintering (SLS) ofpowders (e.g. polymers, resin coated metal and sand) [1] and (c) layerobject manufacturing (LOM) [2] which involves the laser cutting andadhesive bonding of sheets of papers or slip casting of ceramic tapes.These methods have been used to produce prototypes of new designs forevaluation and testing. In addition, they have successfully producedpatterns for casting or plastic molding tools needed for themanufacturing of prototypes of design products or short run products.

However, the parts that are made by these methods always suffer from lowmechanical strength due to poor density or excessive resin, and sorequire additional process steps such as debinding of resin, and heattreatment for densification. This may lead to undesirable shrinkageresulting in poor dimensional control. In addition, these methods arelimited to a selected range of materials—generally being limited to useonly with polymers.

In the SLA method, the monomers are photosensitive and have a lowstorage lifetime, leading to a high cost of the precursor materials.

In the SLS method, the surface finish quality of the final part isconstrained by the size of the powder, and often a subsequent machiningstep is needed to give an acceptable surface finish.

In the LOM technique, the dimensional control is limited due tostaircase-edge effect caused by the stacking of the sheets.

There is therefore a need for a rapid prototyping technology capable ofproducing fully dense and functional parts directly from acomputer-aided-design (CAD) model without the need for the time wastingintermediate processing steps. There is also a need for a techniquewhich will allow other, possibly stronger or otherwise more suitable,materials to be used in rapid prototyping.

Reference [5] discloses a fabrication method involving photocurableplastic sheets, whereby regions of the sheets are cured by applicationof a laser beam. A technique for producing 3-D objects from a powderprecursor has been proposed [6], with a vibration wiper blade being usedto help apply the powder to a recipient surface. A stereolithographymethod involving polymer resin cured by a laser beam [7] uses anelectric or magnetic field used to align particles within the polymer.Fabrication techniques in [8] and [9] make use of a gaseous precursor.

A fabrication method in which an object is formed as a plurality ofsuccessive laminae, the method comprising the repeated steps of:

(i) applying a sol precursor onto a recipient surface;

(ii) providing a temperature gradient along an application path of theprecursor, so that the precursor is heated as it approaches therecipient surface and a sol-gel transition is initiated so that a gellayer is deposited on the surface; and

(iii) locally heating regions of the gel layer by directing a light beamonto those regions of the deposited precursor, so that the locallyheated regions transform to a solid material.

This invention also provides fabrication apparatus for fabricating anobject as a plurality of successive laminae, the apparatus comprising:

(i) a precursor outlet for applying a sol precursor onto a recipientsurface;

(ii) a heater for providing a temperature gradient along an applicationpath of the precursor, so that the precursor is heated as it approachesthe recipient surface and a sol-gel transition is initiated so that agel layer is deposited on the surface; and

(iii) a directable light beam for locally heating regions of thedeposited precursor so that the locally heated regions transform to asolid material.

This invention provides a new rapid prototyping process (embodiments ofwhich will be referred to herein as laser gel manufacturing (LGM))capable of efficiently producing high quality engineering parts,especially ceramic components for manufacturing industries.

The invention will now be described by way of example with reference tothe accompanying drawing, throughout which like parts are referred to bylike references, and in which:

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic diagram of a rapid prototyping apparatus.

Referring to FIG. 1, a “sol” precursor solution is supplied to a nozzleor outlet 10, from which it is sprayed onto a micro-positioning table 20(which may comprise a conventional PZT stage and control electronics—notshown). A voltage source 30 is used to generate an electric fieldbetween the nozzle 10 and the table 20, so that droplets of theprecursor solution emerging from the nozzle 10 are attracted towards thetable 20. Also, a temperature gradient is maintained (e.g. by heatingelements surrounding the table 20—not shown), so that the table is at ahigher temperature than the nozzle 10. Under the influence of thetemperature gradient and guided by the electric field, a sol-geltransition takes place and a gel layer is thus deposited onto the table.

Examples of suitable precursor solutions are as follows:

Sol precursor for the deposition of stabilised zirconia (Y₂O₃—ZrO₂):

Precursor for yttria stabilised zirconia: Y(O₂C₈H₁₅)₃ and Zr(OC₄H₉)₄

Solvent: propanol or butanol catalyst: ethanoic acid

Sol precursor for the deposition of alumina (Al₂O₃):

Precursor for alumina: Al(OC₄H₉)₃

Solvent: butanol and water catalyst: ethanoic acid or hydrochloric acid

A sol to gel transition can occur below 400° C., depending on the typeof precursor and the solvent used. With the above examples, a sol to geltransition can occur between room temperature and about 250° C.

A galvoscanner 40 (a known electrically-driven mirror capable of fine,accurate and rapid motion) causes a laser beam 45 from a laser source 50(focused if necessary by focusing optics 60 shown schematically inFIG. 1) the laser beam to move in the x-y direction (i.e. substantiallyin the plane of the table). Where the laser beam locally heats thedeposited gel, that area of the gel crystallises into a ceramicmaterial. In this way, a section of a solid object is being constructedas a result of the laser-gel interaction. In general, a coherent lightsource with a wavelength between ultraviolet and infrared may be used.

The galvoscanner is shown schematically, but of course would comprise(as well as at least one reflector or other directing optics such aslenses) a motor, a drive mechanism and a controller (e.g. a controlcomputer) forming an overall drive means (not shown). In alternativeembodiments, the beam could be directed by reconfiguring the relativepositions of the directing optics.

The next section is built by lowering the table in the z-direction,followed by the deposition of the gel and the scanning of the laser beamonto the selected region of the gel, to construct the desired geometrythrough the laser-gel interaction. The above sequence is repeated toconstruct a 3-D object.

In addition, the LGM process can be applied to printing and markingapplications, in which the gel material changes its optical properties(e.g. reflectivity and absorption coefficient) as a result of thelaser-gel interaction, after it has been irradiated by the laser beam.The precursor used to form the gel in an electric field can be chemicalsthat are soluble in organic solvents or inorganic solvents. Thechemicals can be natural dyes, metal alkoxides, nitrates, etc.

The process combines the technologies of sol-gel deposition [3],deposition techniques described in our copending application [4] andlaser heating. It involves the preparation of sol and subsequentspraying of the sol across an electric field under a temperaturegradient onto an object forming table to produce an uniform gel depositvia a sot-gel transition induced by laser and/or other heating methods(e.g. by the temperature gradient mentioned above or by heating thenozzle 10). Subsequently, an intense laser beam is used to causelocalised gel-ceramic phase transformation at selected regions of thegel deposit. A 3-D ceramic part can be constructed layer-by-layerthrough the manipulation of the movement of laser beam and objectforming table. The laser beam scans the gel deposit on the objectforming table according to the tool paths¹, thereby forming a crosssection of an engineering part via gel-ceramic transformation. Theobject forming table is then moved one layer down to allow the nextlayer of gel to be deposited and selective regions of the gel isconverted into the ceramic phase at selective regions using a scanninglaser beam. The 3-D part is then constructed layer-by-layer with thefinest details until the final object is formed.

¹ A 3-D computer aided design model of an engineering part is slicedinto many 2-D sections. The geometries of each section are used togenerate the tool paths to move the laser beam in a specific mannerusing a computer numerical control (CNC) system.

The time taken to build a volume of engineering parts is influenced bythe processing parameters (such as sol composition, gel deposition rate,laser energy density, laser scan speed, scan spacing and tabletemperature and deposition rate, etc.). This method can reduce theamount of undesirable impurities in the engineering parts because itavoids the use of binder/sintering aids to provide the mechanicalstrength of the part during or after building. This process can be usedin atmospheric or reduced atmospheric pressure.

Embodiments of this fabrication method can therefore combine thebenefits of both laser aided transformation and the deposition processesof reference [4], offering the following advantages:

(1) good dimensional control due to a small volume change duringprocessing

(2) excellent surface finish

(3) rapid manufacture of highly dense and/or porous engineering parts

(4) no support is required during the net shape forming process

(5) ease of extraction of parts after the end of the manufacturingprocess

(6) process can occur in open atmosphere for the production of ceramicoxide parts

(7) application to a wide range of ceramic materials e.g. Al₂O₃,Y₂O₃—ZrO₂ etc.

(8) simple, multicomponent oxides and doped-oxides parts can bemanufactured with well controlled structure and composition.

(9) non-oxide components (such as those of metal, polymer andcomposites) can be manufactured in controlled atmosphere.

(10) simple/flexible equipment

(11) low cost and safe process using water soluble or organic solubleprecursors

(12) manufacturing process can occur at low temperatures, e.g. 300-600°C. for stabilised zirconia based ceramic parts

(13) one-step processing without the need for further heat treatment

(14) possible to manufacture small, and precise engineering parts, aswell as large engineering parts.

In place of the sol precursor, aerosol or gaseous precursors could beused. In this case a similar apparatus would be used. The aerosol orgaseous precursor is passed through the nozzle or outlet 10 and directedonto the micro-positioning table 20. The voltage source provides anelectric field so that the precursor is attracted towards the table.Local heating (by the light beam) forms areas of solid stable material.This sequence is repeated to produce a 3D object. Additional heatingsources can be used to heat the object if required. The approach can beused to manufacture oxide and non-oxide parts. For example, in thefabrication of B4C parts, the gaseous precursor can comprise a mixtureof boron trichloride, hydrocarbon and/or hydrogen gases. The gases canalso be fed separately through a coaxial cylindrical nozzle.

Besides liquid precursors, the process and apparatus of FIG. 1 can alsoaccommodate solid precursors such as powder for the manufacture of oxideand nonoxide parts. A carrier gas may be used to facilitate thetransport of powder in the nozzle.

The process is not necessarily fixed within a confined space, but can bemade portable so as to allow the manufacturing of large componentsand/or for repairs. Thick or thin films can be deposited onto planarsubstrates or onto substrates with complex geometry.

The process may be preformed in an open atmosphere for oxide materialsor in a controlled atmosphere for non-oxide materials. The process canbe performed at atmospheric or low pressure.

PUBLICATION REFERENCES

[1] T Grimm, Rapid News, Vol 4(4), p48, (1996)

[2] E A Griffin, D R Mumm and D B Marshall, American Ceramic SocietyBulletin, Vol 75(7), p65 (1996)

[3] C J Brinker and G W Scherer, Sol-Gel Science, Academic Press Inc(1990)

[4] PCT Patent Application number PCT/GB96/03105

[5] EP-A-0 467 097

[6] DE-A-4 325 573

[7] WO-A-93/20993

[8] WO-A-93/02846

[9] WO-A-92/16343

We claim:
 1. A fabrication method in which an object is formed as aplurality of successive laminae, the method comprising the repeatedsteps of: (i) applying a sol precursor onto a recipient surface; (ii)providing a temperature gradient along an application path of theprecursor, so that the precursor is heated as it approaches therecipient surface and a sol-gel transition is initiated so that a gellayer is deposited on the surface; and (iii) locally heating regions ofthe gel layer by directing a light beam onto those regions of thedeposited precursor, so that the locally heated regions transform to asolid material.
 2. A method according to claim 1, in which the precursoris a liquid precursor, step (i) comprising spraying the liquid precursoronto the recipient surface.
 3. A method according to claim 1, in whichthe gel layer crystallises to form a ceramic material when heated by thelight beam.
 4. A method according to claim 1, in which the object isformed on a movable substrate, the method comprising the repeated stepof: (iv) moving the substrate so that the most-recently-depositedprecursor then forms a recipient surface at substantially the sameposition with respect to the application of precursor.
 5. A methodaccording to claim 1, in which the light beam is a coherent light beamwith a wavelength ranging from ultraviolet to infrared.
 6. A methodaccording to claim 5, in which the light beam is a laser beam.
 7. Amethod according to claim 1, comprising: applying an electric fieldbetween a precursor outlet and the recipient surface, to attract theprecursor towards the recipient surface.